For example, the Patent Document 1 discloses a rolling die for producing external threads such as bolts or male screws made of the material having a high degree of hardness through a rolling process.
Specifically, there is shown a rolling die having a bite portion in which processing teeth at a starting point in the bite portion have a lower tooth height, a larger tooth tip angle, and a larger trough angle between the processing teeth at the starting point than processing teeth in a finishing portion. In the rolling die described above, since the processing teeth at the starting point in the bite portion are shallow and have an obtuse angle for the initial bite at a workpiece or blank, cracking or localized breakage becomes less likely to occur near the tooth tip at the bite portion that includes the starting point, and a longer life of the rolling die can be allowed.
Recently, an effort is underway to reduce the weight of products such as automobiles, and there is increasing demand for heat-treated bolts having a high degree of hardness to be used in such products.